In the reproduction of originals by transfer of an electrostatic image, an electrostatic latent image is formed upon a photoreceptor or an electrostatic film. Fine powder, called a toner, comprising a coloring material dispersed in a binder resin is adhered to the latent image to enable it to be seen. The resulting toner image is transferred to a transfer substrate such as paper. The transferred image is then fixed, for example, by applying heat, to thereby obtain a copy. Fixing of the toner image may be effected by solvent treatment or top coating treatment, but thermal fixing is generally employed, in which the toner image is heated to a temperature sufficient to fuse the toner onto the substrate. With the recent increase of copying speed, a so-called heat roll fixing technique is commonly used, comprising passing a substrate having a toner image formed thereon through a pair of rolls composed of a heat roll and a press roll to effect heat pressing.
The heat roll fixing is considered an extremely effective method for toner fixing because it achieves higher thermal efficiency with reduced heat dispersion to rapidly accomplish fixing as compared with other thermal fixing methods. However, since the heat roll or press roll directly contacts the toner image, a part of the toner on the substrate may adhere to the roll, which may cause an offset phenomenon.
The offset phenomenon has been dealt with by coating the roll surface with a releasant, such as fluorine-containing resins, and supplying an offset inhibiting liquid, such as silicone oil, to the surface of the coated roll. This countermeasure, though effective to prevent the offset phenomenon, is accompanied by disadvantages, such as requiring an additional apparatus for supplying the offset inhibiting liquid to the roll surface at a constant feed rate, oil leaks may occur, and the offset inhibiting liquid gives off an odor upon heating.
Hence, attempts have recently been made to carry out heat roll fixing without using an offset inhibiting liquid as described, e.g., in JP-B-53-5549 (the term "JP-B" as used herein means an "examined published Japanese patent application"). Various improvements in toner particles per se have also been proposed for the purpose of eliminating the necessity of the offset inhibiting liquid as described, e.g., in JP-B-52-3304, JP-B-58-58664, JP-A-56-154740 and JP-A-57-8549 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
In cases where use of an offset inhibiting liquid is omitted by incorporating a polyalkylene resin in a toner composition as proposed in the conventional methods as described JP-B-52-3304, the toner in which the polyalkylene resin is merely incorporated shows a narrow range of temperature at which fixing is feasible (fixing latitude).